1. Field of the Invention
This invention relates to extrudable polyorganosiloxane compositions. More particularly, this invention relates to polyorganosiloxane compositions capable of being extruded as unsupported profiles such as tubing that retain their shape during curing at elevated temperatures.
2. Background Information
Unsupported extruded profiles of silicone rubber, such as tubing, are typically formed from semi-solid or solid rubber products referred to in the art as "gums" or "gum stocks." The gums are, in turn, usually obtained by blending a polydiorganosiloxane having a viscosity of at least 1000 Pa.s at 25.degree. C. with a reinforcing filler such as silica, a curing agent such as an organoperoxide and any other desired additives on a heated two- or three-roll mill. The resultant sheet of uncured rubber is then fabricated into the desired shape and subsequently cured. To form extruded profiles, a sheet of uncured rubber is typically cut into strips prior to being fed into the barrel of an extruder wherein the rubber is subjected to sufficient heat and pressure to force it through a die of the required configuration. The resultant profile is then cured by heating it at temperatures above 100.degree. C.
The use of gums to form extruded silicone rubber profiles has several disadvantages, including the high cost of the equipment and energy required to process the gum in sheet form, cut the sheet into strips and feed the strips into an extruder capable of generating sufficient pressure to force the plasticized material through the die.
The prior art, for example, U.S. Pat. No. 4,162,243, issued to C. L. Lee et al. on July 24, 1979, and U.S. Pat. No. 4,427,801, issued to R. Sweet on Jan. 24, 1984, disclose extrudable, heat curable silicone rubber compositions exhibiting viscosities ranging from pourable liquids to a consistency resembling petrolatum. All of these compositions will flow at a temperature of 25.degree. C. under pressure, and can therefore be transferred from one location to another by pumping. The use of such pumpable compositions for the fabrication of extruded articles avoids the aforementioned processing disadvantages of gum type silicone rubber.
Attempt to cure unsupported profiles such as tubing formed from heat curable, high consistency, pumpable silicone rubber compositions of the type described in the aforementioned patents to Lee et al. and Sweet have been unsuccessful because the profiles do not retain their initial configuration at the temperatures required to cure the compositions at practical rates using a platinum-catalyzed hydrosilation reaction.
Pumpable polyorganosiloxane compositions typically include finely divided silica as a reinforcing filler and a silica treating agent to prevent a phenomenon referred to as "creping" or "crepe-hardening." Creping is believed to result from an interaction between high molecular weight polydiorganosiloxanes and reactive sites present on the surface of the silica filler. This interaction can result in a substantial increase in the viscosity of the composition to the extent that the composition cannot be readily fabricated. The silica treating agent is usually a relatively low molecular weight hydroxyl endblocked polydiorganosiloxane or a hexaorganodisilazane.
The prior art teaches that the hydrocarbon radicals present in the silica treating agent should be identical to, or at least selected from the same class as those in the polydiorganosiloxane gum or other high molecular weight polymers present in the composition. Liquid, high molecular weight polydimethylsiloxanes are not miscible with silica treating agents wherein more than about 40 percent of the silicon-bonded hydrocarbon groups are octadecyl, phenyl or 3,3,3-trifluoropropyl, making it difficult to prepare a homogeneous silica filled composition containing these ingredients.
The addition of small amounts of immiscible fluorine-containing organosilicon materials as silica treating agents to increase the rate at which highly viscous, marginally processable polydimethylsiloxane compositions can be extruded under a specified set of conditions is disclosed in U.S. Pat. No. 4,500,659, issued to Kroupa and Relyea on Feb. 9, 1985. The concentration of immiscible treating agents is up to 4 weight percent, based on silica weight. Data in this patent demonstrate that above this concentration level the maximum rate at which a composition can be extruded under a given set of conditions was comparable to a control that did not contain the immiscible additive. The advantage of using the immiscible treating agent is no longer apparent.